It is known to use products with a blown, foam structure when a product with a certain volume and a minimum weight is desired, for instance for packaging purposes such as in packages configured for receiving and holding fragile, breakable products. Such packages may be manufactured from a foamed plastic such as expanded polystyrene or expanded polypropylene. A drawback of using such material as packaging material is that it is not particularly environmental friendly because the difficulty to recycle or dispose it and because such material is made from non-renewable resources.
A more environmental friendly alternative is provided by means of manufacturing such products from a bio-polymer material from renewable resources, such as a starch based material. Such a method is for instance known from WO 96/30186 and from U.S. Pat. No. 6,521,147. The known method comprises supplying a starch based starting material comprising water as blowing agent under pressure into or through a mould and heating said mass in the mould in a manner such as to give rise to gelatinization and cross-linking of the natural polymers. Prior to the introduction in the mould, the mass has a temperature which is at or below the gelatinization temperature and in the mould the mass is maintained at least for some time at a temperature which is above the baking temperature. In this method, the mould cavity is filled with the mass for approximately 50%. Due to the stabilization inside the mould cavity and the heat activation of the blowing agent, the mass is going to create foam, the foam creates additional volume and force to flow further into the cavity, and in this way, the complete cavity is filled. The mass may be a liquid batter comprising a suspension or solution of at least natural polymers such as starch in a liquid. By using such a batter, which is liquid below the gelatinization temperature, supply of the batter can be realized in a simple manner, for instance via pipes and using simple pumping means. The liquidity of the batter provides the advantage that the flow path in the mould is particularly long. The water in the batter functions as blowing agent and moreover, upon evaporating from the mould, provides space for the expansion of the cells of the polymers. Alternatively, the mass may be in a dry condition when introducing it into the mould. The mass may for instance be a mass in a granulate form and comprise more or less spherical particles having small to very small dimensions with respect to the passage openings to and in the mould. This granulate material can contain a blowing agent, for instance in the form of water or blowing agents simply released and/or evaporating upon heating such as bicarbonates which provide for gas evolution through decomposition at elevated temperature. The granulate particles may be slightly prefoamed prior to introduction in the mould without this giving rise to gelatinization. Gelatinization is intended to refer to a change of a natural polymer form a slightly or completely loose granular or comparable granulate form into a dry or non-dry and/or foamed cohesive form, in which stretched polymers are present which are mutually bonded to a limited extent only, if at all. The use of relatively dry, optionally slightly pre-foamed starting material provides the advantage that relatively little water or other moisture needs to evaporate in the mould. The granulate-form mass may be used to form a product by means of for instance extrusion on an extrusion apparatus. Such apparatus therefore may comprise an extrusion die and supply means for feeding a mass under pressure through said die. The apparatus further comprises heating means for heating the die and means for keeping the supply means relatively cool with respect to the die to keep the mass inside the supply means at least below the gelatinization temperature. The mass in granulate form, whether or not in a slightly pre-foamed condition is fed from a supply device of the apparatus via the spray nozzle to the heated extrusion die and forced through the or each extrusion orifice. The granulate particles can contain for instance, water or a different blowing agent in a relatively small amount. The particles are heated after being introduced into the die and will swell as a result since the blowing agent present will blow up the particles, just as in the case of the particles included in a liquid batter The leading part of the mass gelatinizes and proceeds to cross-link, whereby the moisture evaporates from the mass and provides for the foaming of the product.
During insertion of the mass in a mould cavity according to WO 96/30186, formation of a skin, i.e. at the contact surface of the mass and the mould cavity, is started immediately. Consequently, at different locations of the product outer surface, depending on the flow of the mass inside the mould cavity, the skin is formed at different moments. This may lead to different surface structures at the outer surface of the product, for instance areas with a smooth surface and areas with a more structured surface. Especially, when manufacturing products having a relatively dark colour this may lead to products with a marbled outer surface, which may be aesthetically undesired for instance dependent on the specific function of the product. Besides, due to fact that the actual foaming of the mass takes place inside the mould cavity, correct dosing of the mass that is inserted in the mould cavity may be difficult. Furthermore movement inside the mould cavity of initially formed and dried skin may result in obstruction of one or more deaeration channels present in the mould. Also the foaming process inside the mould cavity may be difficult to control.
Thus, it is an object of the present invention to provide an improved method for manufacturing products with a blown, foam structure based upon renewable biopolymers. More in particular an object to the invention is to provide a method for manufacturing products with a blown, foam structure, in which the skin formation and the internal structure of the foam on different locations in the mould cavity may be more evenly distributed and in which the mass insertion process may be easier to control.